Apparatus for treating lime mud

ABSTRACT

A method for feeding lime mud into a lime kiln including a rotary kiln shell having an interior between a first end wall and a second end wall, the method including: feeding the lime mud into a flue gas flow in the interior of the rotary kiln shell or in close proximity to the shell to pretreat the lime mud; separating the pretreated lime mud from the flue gas flow; conveying the separated lime mud into the lime kiln, and calcining the separated lime mud in the lime kiln.

BACKGROUND OF THE INVENTION

The invention relates to a method and apparatus for treating lime mud,and a particular configuration of a rotary lime kiln at a pulp mill. Alime kiln is part of the pulp mill chemical recovery plant, the limebeing used for causticizing green liquor to produce white liquor in theproduction of kraft pulp.

Lime mud is produced in the causticizing plant of a sulphate pulp mill.For reuse the lime mud which is mainly calcium carbonate (CaCO₃) isregenerated by reburning it to form calcium oxide (CaO). The reburningtakes place in an ordinary rotary kiln, into the upper end of which thelime mud is supplied. The mud flows slowly downwardly through drying,heating and reaction zones. Conventional rotary kilns desirably enhancethe heat transfer from the flue gases and the lime mud in the dryingzone by providing chains and/or lifters, which rotate with the kiln andcome into contact with the lime mud during that rotation. In order toobtain proper results the heat treatment of lime mud typically takesplace slowly in the kiln, meaning that the kiln must be long andtherefore takes up significant floor space or land area.

One prior art method to allow a shorter kiln is a suspension-type dryingsystem. In such systems lime mud is introduced into a vertical conduitthrough which the flue gas from the kiln moves upwardly at a relativelyhigh rate of speed. Most of the lime mud is entrained in the upwardlymoving gas, and dried by contact therewith, and the flue gas withentrained particles is fed to a conventional separator (such as acyclonic separator), the flue gas being discharged and the lime mudparticles—which now have been dried—being fed to the inlet to the limekiln. U.S. Pat. No. 5,110,567 discloses thickening of lime mud in a limefilter to a dryness of over 75%, whereby it is possible to introduce thelime mud directly into a suspension dryer without the necessity of usinghammer mills or mixing screws for crushing lumps of lime mud. The limemud is so dry that the dryer will remain open. Another point of feedinglime mud is, however, still needed for the lime reburning kiln. When the“precoat” of the lime mud filter is replaced, it is not desirable tofeed the moist lump of lime mud into the suspension dryer because thereis a risk of the dryer becoming clogged, but the lime mud lump isintroduced directly into the lime reburning kiln by means of a separatefeed screw.

U.S. Pat. No. 5,413,635 discloses a method, in which a controllableamount of flue gas that has exited a flash dryer and has been separatedfrom the dried lime mud is recycled back into the vertical portion ofthe flash dryer below the feed inlet portion of the flash dryer. The aimis that the velocity of the gas through the flash dryer is maintained ata level sufficient to entrain all of the lime mud feed in the gasstream. In a process malfunction, where the gas velocity is sufficient,the moist mud falls down. This may cause plugging.

U.S. Pat. No. 5,213,496 discloses a method for feeding lime mud to alime kiln according to which method all the moist lime mud from a limemud filter is supplied to the upper end of a feed chamber of the kiln.Lime mud may be transported from this chamber either to a suspensiondryer or directly to a kiln or both, depending on the dry solids contentand particle size of the lime mud. There is a partition wall in theupper portion of the feed chamber dividing the chamber into two flowchannels. The amount of lime mud entering the dryer may be regulated bychanging, by means of a control baffle disposed in the upper section ofthe partition wall, the relationship between the gas flow volumesflowing through the adjacent flow channels. In the lower end of the fluegas chamber, there is a spiral feeder, which transports the moist limemud falling into the lower end to the kiln. Dried lime mud from theseparation apparatus is brought via a return duct to the vicinity ofthis spiral feeder. Such systems—namely combinations of suspension-typedryers and rotary kilns—are replacing rotary kilns per se in themarketplace because the combination of a suspension-type dryer androtary kiln provides high heat capacity and good heat economy. Capacityand heat economy of such a combination may be further improved byemploying two subsequent suspension drying stages, one of which servesas a dryer per se, the other acting as a preheater. The flue gases froma kiln are first taken to a suspension preheater and from there to thedryer. The lime mud to be dried is supplied from a lime mud filter tothe dryer, then to the preheater, and finally to the kiln to becalcinated therein.

In U.S. Pat. No. 5,711,802 (European patent No. 751916) it is statedthat a disadvantage of the plant having the above-described drying andpreheating stages is that the temperature of the preheating stage maybecome so high that lime mud tends to stick onto metal surfaces. It isfurther stated that dry sticking problems occur typically attemperatures in the range of 400-600° C., depending on the dry solidscontent of the lime mud. In U.S. Pat. No. 5,711,802, the sticking oflime mud is prevented so that the temperature of the preheating stagedoes not exceed a temperature between 400-600° C. The temperature isregulated either by feeding part of the moist lime mud directly into thepreheating stage or by directing part of the flue gas from the lime kilndirectly into lime mud drying, thus bypassing the lime mud preheatingstage. However, both regulation methods are complicated to accomplish inpractice. The disadvantage of the first-mentioned method is the feedingof moist lime mud into two separate locations positioned far from eachother. This results in both a complicated transporting and feedingapparatus and increasing maintenance load. In the latter regulationmethod, the problem consists in the large amount of flue gastransportation piping and the fact that maintaining an adequate flue gasflow in the piping may in extreme situations (i.e. very moist or verydry lime mud) require special arrangements.

When feeding dried and/or preheated lime mud into the kiln, metallicconstructions of the feed end of the kiln are exposed to severe stressin the gas exit temperatures. The strength of metals begins to decreaseat high temperatures, even though their heat resistance otherwise wouldstay at a reasonable level. Further, at these temperatures lime mudtends to stick onto surfaces. FI patent No. 106642 discloses a method,according to which part of the thickened moist lime mud is dried andpreheated by means of flue gas from the lime kiln, separated from theflue gas and fed into the feed end of the lime kiln. For cooling theconstructions of the feed end of the kiln, part of the moist lime mud isfed directly into the feed chamber of the lime kiln, bypassing the fluegas treatment, in order to cool the feed end constructions.

FI patent No. 108235 discloses a method, according to which moist limemud is dried by means of flue gas originating from lime mud calcinationand separated from the flue gas, and the dried lime mud is preheated bymeans of flue gas originating from lime mud calcination, separated fromthe flue gas and fed into a calcination apparatus. The temperature ofthe preheating is regulated into a certain value in the range of400-600° C. by circulating part of the preheated lime mud into the limemud drying stage.

Even though it may be stated based on the above that several solutionshave been presented for regulating the temperature in lime mud treatmentwith flue gases and at the feed end of the lime kiln, there still existsa need to find a simpler method for controlling the temperature inconnection with lime mud feeding, so that e.g. lime mud sticking can bedecreased or eliminated. Further, the known kilns include a feedchamber/smoke chamber connected to the kiln shell which makes the kilnlonger.

BRIEF DESCRIPTION OF THE INVENTION

With existing lime kilns having a suspension dryer, the temperature ofthe flue gas leaving the rotary lime kiln exceeds approximately 700° C.,and at a temperature somewhere around 700 C., the lime mud stickingbegins to disturb the normal operation of the process, because lime mudmaterial builds up on hot surfaces. Lime mud which is not entrained inthe flue gas falls down and requires additional equipment to transportit into the rotary kiln. Therefore, the capacity of the rotary kiln islimited by the maximum flue gas exit temperature due to build-upproblems described above.

To avoid the above problems associated with the prior art systems limemud may be introduced into the flue gas stream of a lime kiln in such away, which cools the gas stream. This provides for a method by means ofwhich the conditions prevailing at the feed end of the lime kiln aresuch that sticking of lime mud onto hot surfaces and the wearing ofthose surfaces can be efficiently prevented. A further advantage of thetreatment is to simplify the construction of the lime kiln.

A method has been developed for feeding lime mud into a lime kilncomprising a rotary kiln shell having a first and a second end wall,wherebetween an interior of the kiln is formed, according to whichmethod the lime mud is pretreated by means of flue gases from the limekiln by feeding the lime mud into a flue gas flow, the pretreated limemud is separated from the flue gas, conveyed into the lime kiln andcalcined therein. The lime mud to be pretreated is fed into the flue gasflow in the interior of the rotary kiln shell or in close proximity ofthe shell.

An apparatus for treating lime mud has been developed, comprising:

a rotatable kiln comprising a shell having a first shell end into whichlime mud is introduced, and from which flue gases from calcining limemud within a calcination space of the kiln are discharged, and a secondshell end from which calcined lime mud is discharged;

a first separator device for separating pretreated lime mud from fluegases;

a riser duct for simultaneously conveying flue gas from the kiln andlime mud introduced into the flue gas to the separator, said duct havinga first duct end connected via a kiln exit gas duct to the kiln and asecond duct end connected to the separator;

feeding means for dosing lime mud into the riser duct; and

conveying means for conveying pretreated lime mud to the kiln forcalcining therein. A feature of the apparatus is that the feeding meansis arranged so that lime mud to be pretreated is introduced into theriser duct at a point located inside the kiln shell or in closeproximity of the shell.

A “riser duct” is the duct or conduit which conveys the flue gas andlime mud from the point at which the lime mud is introduced into theflue gas to the entrance of the separation device. The lime mud is driedand heated by the heat of the flue gas in the mud riser duct. “Kiln exitgas duct” is a duct or conduit which is connected to the lower end ofthe riser duct and which directs the flue gas flow from the kilninterior to the riser duct. Depending on the location of the point wherelime mud is introduced into the flue gas the kiln exit gas duct islocated partially or wholly inside the kiln shell. The term “closeproximity” can, in one embodiment, mean a distance of half (0.5) of thediameter of the kiln shell or less from the center of the first end ofthe kiln shell to the point at which the lime mud enters the riser ductoutside the kiln shell. The feeding means or feeder for introducing limemud into the flue gas in the riser duct is typically a feed screw orlike, but it can be also a pipe, a combination thereof or other lime mudfeeding device. The conveying means or conveyor for leading pretreatedlime mud to the kiln is typically a pipe, but it can also be adrag-chain conveyor, other conveying devices or combinations thereof.

Moist lime mud is introduced into the flue gas stream of a lime mud kilnin such a way, which cools the gas stream and eliminates or at leastdecreases plugging of the feed end of the kiln and the ducts of thesuspension dryer. Kilns having a suspension dryer plug in the feed endof the kiln if the gas temperature gets above about 700° C. and the limemud gets sticky enough to start coating the walls, spirals, etc. Thismakes the sticking temperature an operational limit if there is no meansto cool the gas quickly. A way to lower the temperature is to use thecold lime mud to drop the gas temperature quickly before it can stick toanything. The new design mixes the lime mud and the flue gas at the endof the riser duct as the gas exits the kiln. The location is actuallyinside the kiln shell or in close proximity of the shell. Known designsintroduce the lime mud into the gas stream well downstream from the feedend housing of the kiln which makes them vulnerable to plugging.

Cooling of flue gases of a lime kiln enables the kiln to operate at asignificantly higher back-end or exit gas temperature without plugging.This further allows higher production rate for any given kiln size. Thelimit of approximately 700° C. on kiln feed end temperature becomes thedesign point used for sizing lime kilns having a suspension dryer. Withthis design, there should be no limit or at least it will besignificantly higher. This will allow reducing the kiln size for a givencapacity. A separate feed chamber/flue gas chamber is not needed anymore. Thus the lime kiln of the present invention is devoid of a smokechamber or feed end housing.

In the design of the feed system, the introduction of lime mud into theflue gas stream can be controlled in such a way that lime mud is eitherentrained into the flue gas stream or a portion of the lime mud fallsdirectly into the interior of the kiln. In the latter case moist limemud does not drop, like in known kiln systems, into a feed end housingof the kiln, where plugging could occur before the lime mud either fallsfurther or is transported into the kiln. Here the cause of plugging isthe dropping of mud from the feed screw into the housing. This isparticularly a problem if the gas velocity in the drying pipe of thesuspension drier is low or the mud is very moist. If these conditionsshould occur with the present design, all of the mud will fall directlyto the interior of the kiln and no plugging will occur. In the presentdesign there is no equipment required between the lime mud feed pointand the point at which the flue gas enters the riser duct so that, inparticular, smoke chambers, feed end housings and other equipment onwhich lime mud can build up have been eliminated. Lime mud which is notentrained may fall directly into the lime kiln without the need foradditional equipment to transport the lime mud. Lime mud can be bypassedbefore it is fed to the flue gas stream and can fall directly into thelime kiln without the need for additional equipment to transport thelime mud.

With existing lime kilns having a suspension dryer, the lime mud filteris located typically one floor level higher than the feed end of thelime kiln, because the lime mud has to be introduced into the kiln fluegas in the riser duct above the smoke chamber or feed end chamber. Thedesign eliminates this limitation, because the lime mud to be pretreatedcan be introduced into the flue gas even inside the kiln. Thus the limemud filter can be standing on the same floor level as the feed end ofthe lime kiln. This allows considerable space savings in verticaldirection.

Preferably moist lime mud from a lime mud filter is introduced into theflue gas stream in the riser duct. According to another embodiment ofthe design, the kiln has two subsequent suspension drying stages, one ofwhich serves as a dryer per se, the other acting as a preheater. Theflue gases from the kiln are first taken to a suspension preheater andfrom there to the dryer. The lime mud to be dried is supplied from alime mud filter to the dryer, then to the preheater. According to thisembodiment the pretreatment comprises two stages so that in a firststage moist lime mud thickened in a lime mud filter is dried by means offlue gas from the kiln and separated from the flue gas, and in a secondstage dried lime mud from the first stage is preheated also by means offlue gas from the kiln, separated from the flue gas, whereby in thesecond stage dried lime mud is introduced into the flue gas flow in theinterior of the rotary kiln shell or in close proximity of the shell.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described more in detail below with reference to theaccompanying drawings of which,

FIG. 1 is a schematic principle illustration of an exemplary apparatusaccording to the present invention.

FIG. 2 is a side view, in cross-section, of a lime kiln inlet, accordingto a first embodiment of apparatus according to the present invention.

FIG. 3 is a side view, in cross-section, of a lime kiln inlet, accordingto a second embodiment of apparatus according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The main parts of the apparatus shown in FIG. 1 comprise a feed screw 1,a lime kiln 2 and a riser duct 3 in connection thereto and a dryerseparation cyclone 4. The lime kiln comprises an elongated kiln shellhaving a feed end wall 7 and a discharge end wall 6. The interior 8 ofthe kiln is located between the end walls.

The kiln is devoid of a separate feed chamber/flue gas chamber inconnection with the kiln, but the lower part 9 of the riser duct 3 islocated in the interior 8 of the kiln, and in this case the lower end ofthe riser duct forms also a kiln exit gas duct. Flue gases from the limekiln flow upwards via this duct into the separator cyclone 4. Lime mudis thickened in a lime mud filter 16 which is standing on the same floorlevel as the feed end 7 of the kiln 2. The lime mud is thickenedtypically to a dry solids content of over 75%, but the dry solidscontent can also be less than 75%. There is a conveyor belt 17 whichdrops the moist lime mud coming from the filter via pipe 10 to a feedscrew 1, or a bull chain conveyor or a corresponding transporter. Thefeed screw doses the lime mud directly into the interior 8 of the kilninto the lower part 9 of the riser duct, wherein the lime mud is eithertotally or partly entrained in the flue gas flow and transported withthe flue gas into the separator 4 and simultaneously it dries under theeffect of the flue gas heat. The dried lime mud is separated from theflue gas, discharged from the separator 4 and taken via a pipe 5connected to the dryer's bottom end into the interior of the lime kilnto be calcined. The pipe 5 serves as a feeder and conveyor of the limemud. In screw 1 dried lime mud may also be mixed into moist lime mudbeing fed into the kiln and further into the riser duct 3.

According to an alternative embodiment, lime mud can be introduced intothe kiln flue gas outside the kiln. In FIG. 1, moist lime mud enteringvia pipe 10 a is directed by a screw 1 a into the riser duct 3. Theintroduction point of lime mud is located in close proximity of the kilnshell 14.

The invention is not limited to the embodiments illustrated in FIG. 1,but in addition to a dryer, also a lime mud preheater may be connectedto the lime kiln, whereby in the riser duct connected to the kiln limemud is treated with hot flue gas, which lime mud has previously beendried by means of flue gas discharged from the preheating. The preheatedlime mud is separated from the flue gas in a separator, wherefrom it istaken into the lime kiln for calcination.

FIG. 2 illustrates a feed end construction of a lime kiln according tothe invention. In this solution, the lower part of riser duct 3 islocated in the interior of the kiln.

Part of the endwall 7 of the kiln shell is formed of an end shield 11,which is stationary, whereas the other part 7 a rotates. The lower end 9of riser duct 3 is located a distance within the kiln shell. The riserduct has a vertical portion, below which there is an elbow and the lowerpart of the riser duct leading aslope into the kiln shell.

Flue gas generated in calcination is led into riser duct 9, which alsoreceives lime mud from drop tube 10 by means of feed screw 1. The limemud is suspended in the flue gas in the riser duct and transferred withthe flue gas into cyclone 4. The lime mud separated from the flue gas inthe cyclone is led via pipe 5 into the kiln for calcination. In thissolution, the lime mud is introduced into the flue gas in the riser ductin a point located in the calcination space in the interior 8 of thekiln.

FIG. 3 illustrates a more preferable embodiment, wherein the exposure ofthe riser duct and the lime mud feeding device to hot flue gases hasbeen decreased. The feed end of the kiln shell is provided with asection 12 isolated from the calcination space of the kiln by means of aheat-resistant dividing wall 15, e.g. brickwork. The lime mud feed screwends up into this space, wherein it feeds the lime mud into the riserduct 3. The lower end of the riser duct, or to be precise, the kiln exitgas duct extends into the calcination space of the kiln only to such adistance that is adequate to catch flue gas into the duct. Thisstructure can be called an inverted feed housing, because the spacewhich protects lime mud treatment devices from hot flue gases is nowlocated inside the kiln shell.

The kiln exit gas duct is very short, whereas in the known lime kilnssmoke chambers and long ducts or conduits are needed for leading fluegas from the kiln shell to the riser duct. In particular smoke chambers,feed end housings and other equipment on which lime mud can build uphave been eliminated in the present invention.

In connection with the feed screw, here below it, a regulating device isprovided, by means of which part of the lime mud being conveyed by thefeed screw 1 can, if needed, be directed directly into the kiln shell.The figure illustrates a slide gate 13, the opening of which may bechanged to make a desired portion of the lime mud to fall directly intothe kiln. The regulating device can also be a rotary valve or tippingvalve or like.

As can be seen the method and apparatus disclosed herein is highlyadvantageous, substantially allowing an immediate mixing of lime mud andhot flue gas thus resulting in quick gas cooling and preventingplugging. Thus the kiln exit gas temperature can be substantially higherthan in conventional kilns, which means that a shorter kiln for the samecapacity can be used. This allows the utilization of a feed endstructure which is lighter than in conventional kilns, because the limekiln is devoid of a feed end housing located exteriorly of the kilnshell, all lime mud is fed by means of a single feeding device, such asa screw. If needed, such as in disturbances, lime mud falls directly tothe kiln shell. The kiln structure is easier to clean.

1. An apparatus for treating lime mud comprising: a rotatable kilnincluding a kiln shell having a center axis, a first shell end intowhich lime mud is introduced and from which flue gas from calcining limemud within a calcination space in the kiln shell is discharged, and asecond shell end from which calcined lime mud is discharged; a firstseparator device for separating pretreated lime mud from the flue gas; ariser duct simultaneously conveying the flue gas and lime mud introducedinto the flue gas to the separator, said riser duct having a first ductend connected via a kiln exit gas duct to the kiln and a second duct endconnected to the separator; a feeder adapted to dose lime mud into theriser duct, wherein the feeder is arranged so that lime mud to bepretreated is introduced into the riser duct at a point located insidethe kiln shell or in close proximity to the kiln shell, and a conveyoradapted to convey pretreated lime mud to the kiln for calcining therein.2. An apparatus in accordance with claim 1 wherein the first duct end ofthe riser duct is located inside the kiln shell such that the feederdoses lime mud into the flue gas inside the kiln shell.
 3. An apparatusin accordance with claim 2 wherein the riser duct and the feeder arearranged such that the lime mud is entrained in the flue gas flow in thecalcination space of the kiln shell.
 4. An apparatus in accordance withclaim 2 wherein the kiln shell is provided with an isolated section inthe first kiln end, the isolation section being isolated from thecalcination space by a dividing wall in the kiln shell, and the riserduct and the lime mud feeder are disposed such that the lime mud isentrained in the flue gas flow in the isolated section.
 5. An apparatusin accordance with claim 1 wherein the feeder is arranged to dose limemud to the riser duct at a point located at a distance of half of thediameter of the kiln shell or less from a center of the first shell end.6. An apparatus in accordance with claim 1 wherein the lime mud feederincludes a regulating device that guides at least one portion of thelime mud directly to an interior location in the kiln shell before amixing point of the lime mud with the kiln flue gas.
 7. An apparatus inaccordance with claim 6 wherein the regulating device includes at leastone of a slide gate, a rotary valve and tipping valve.
 8. An apparatusin accordance with claim 1 wherein the feeder is connected to a lime mudfilter supplying moist lime mud to the riser duct.
 9. An apparatus inaccordance with claim 8 wherein the lime mud filter is locatedsubstantially at a same level as the feed end of the lime kiln.
 10. Anapparatus in accordance with claim 1 further comprising a further ductand a second separator device treating lime mud with kiln flue gasbefore supplying to the riser duct and the first separator device.